Embodiments of the invention relate generally to inertial sensor gyro-compassing and, more particularly, to an electrostatic carouseling technique for inertial sensor gyrocompassing.
Navigation or “north-finding”—where the orientation of a moving object is determined—plays a crucial role in many different industries and technology applications, including aviation, downhole drilling and mining, and the like. Not only is north-finding of importance in these existing areas, but with the recent explosion of use of personal navigation and smart devices that provide navigational capabilities, the role of navigation is expected to expand to many additional industries and technology applications, with consumer drones being an example of one such area.
North-finding is traditionally accomplished through the use of the magnetic field of the earth; however, there are a number of spatial and temporal distortions in this field due to varying magnetic background, which limit the accuracy of magnetic compasses. Moreover, practical limitations of geodetic, celestial, and GPS-based methods make high performance gyroscopes desirable for true north finding. A method of north-finding based on gyroscopes is commonly referred to as “gyrocompassing.” Although commercially available macro-scale fiber optic, ring laser, dynamic tuning gyroscope (DTG), and quartz hemispherical resonator gyroscopes can be used for precision gyrocompassing, they are not suitable for man-portable and small platform applications. Additionally, these types of gyroscopes require a rotation stage to find the north direction. Accordingly, it has become increasingly popular to utilize microelectromechanical systems (MEMS)-based inertial sensors for gyrocompassing, as MEMS-based inertial sensors provide a number of inherent benefits, including being light-weight, low-power, batch-fabricated, and potentially capable of high performance operation, given the proper design.
In gyrocompassing, inertial sensors find the geographic direction, e.g., north or east, through the sensing of the Earth's rotation and gravity vector. A horizontal angle between the observer and north direction is defined as the azimuth, α, and is measured from north in a clockwise direction, e.g., north is 0° and east is 90°. For such applications as land and aerial navigation, mining, or military, azimuth defines the horizontal direction of heading, pointing or targeting, respectively. Desired azimuth accuracy, σ, in the range from 1 mrad to 4 mrad is often dictated by the requirements of the specific application in order to achieve a target location error, d, which may be in the range of 1 m to 4 m for each 1 km distance of travel, for example.
One technique that may be utilized to identify or detect the azimuth angle is “carouseling.” in carouseling, the gyroscope platform is continuously rotated around the axis that is vertical to the horizontal plane in order to change the horizontal orientation of the gyroscope's sensitive axis with respect to the north, making possible determination of the azimuth angle. The continuous rotation of the platform allows identification of the azimuth angle independently of bias and scale-factor errors. Specifically, the platform rotation causes a variation of angle between the Earth's rotation axis and the gyroscope input axis, leading to a modulation of the gyroscope output by the platform rotation that is independent of gyroscope bias errors. The output is maximum when the gyroscope is pointing north, and minimum when it is pointing south. The sinusoidal fit to the gyroscope output is performed to extract the phase, which is a measure of heading. For each 360° turn, the azimuth angle is calculated by subtracting a phase of the fit from instantaneous position of the platform. At the same time, amplitude demodulation at the frequency of the applied rotation allows to extract time-varying bias and scale-factor independently of the azimuth (phase) measurements.
Another related technique that may be employed to identify or detect the azimuth angle is “maytagging.” In maytagging, the gyroscope platform is rotated around the axis that is vertical to the horizontal plane to multiple pre-defined angular positions (e.g., 0°, 45°, 90° 135° . . . 360°, etc.) and caused to dwell on those positions for a certain duration to allow the gyroscope to collect more data points. The same sinusoidal fit and phase extraction algorithm (as used for carouseling) can be used to determine the azimuth for the maytagging operation. Hereafter, carouseling and maytagging are generally referred to together as “carouseling.”
While carouseling is robust to bias, scale-factor, and temperature drifts, existing MEMS-based inertial sensors require a rotary platform with slip rings and a drive motor in order to provide for the continuous rotation needed for carouseling. The use of such slip rings and motors in inertial sensors has a number of drawbacks/limitations, including cost, size and reliability of the sensor. That is, physically rotating the platform requires a motor, controls, a cable assembly, etc., all of which significantly increase system cost. Additionally, while MEMS technology allows for the gyroscope to be made very small, the motor and the controls needed for rotation of the platform are much larger than gyroscope itself, therefore limiting the applications of the inertial sensor in environments with space constraints. Still further, it is recognized that the use of rotating components and slip rings can lead to reliability issues for the inertial sensor (as such components are prone to failure and wear) and that inclusion/positioning of the rotating components around the gyroscope axis of sensitivity may result in additional rotation being detected by the gyroscope so as to degrade the system performance.
Therefore, it is desirable to provide an inertial sensor that eliminates the need of physically rotating the gyroscope platform for gyrocompassing.